In the manufacture of metal castings it is important to avoid contamination of the metal with non-metallic inclusions. These inclusions are usually oxide phases, and are usually formed by reaction between the metals being melted and the crucible in which they are melted. It has long been an aim of metalcasters to avoid such contamination by using crucibles which have minimum reactivity with the melts. However, some alloys, in particular nickel-based superalloys, which may contain substantial amounts of aluminum, titanium, or hafnium, react vigorously with oxide crucibles and form inclusions during melting.
In the case of titanium-base alloys and alloys of refractory metals (tungsten, tantalum, molybdenum, niobium, hafnium, rhenium, and zirconium), crucible melting is virtually impossible because of the violence of reactions with the crucible. So a related aim of metalcasters is to find a way to melt these alloys without contamination.
Heretofore there have been two main methods of avoiding contamination from a crucible in metal smelting. One method is "cold-crucible" melting, in which a water cooled copper crucible is used. The metal charge, which may be melted by induction, electric arc, plasma torch, or electron beam energy sources, freezes against the cold copper crucible wall. Thereafter, the liquid metal is held within a "skull" of solid metal of its own composition, instead of coming in contact with the crucible wall.
Another method is levitation melting. In levitation melting, a quantity of metal to be melted is electromagnetically suspended in space while it is heated. U.S. Pat. No. 2,686,864 to Wroughton et al. and U.S. Pat. No. 4,578,552 to Mortimer show methods of using induction coils to levitate a quantity of metal and heat it inductively.
Cold crucible melting and levitation melting necessarily consume a great deal of energy. In the case of cold-crucible melting, a substantial amount of energy is required merely to maintain the pool of molten metal within the skull, and much of the heating energy put into the metal must be removed deliberately just to maintain the solid outer portion. With levitation melting, energy is required to keep the metal suspended. In addition, as compared to the surface of a molten bath in a conventional crucible, levitation melting causes the quantity of metal to have a large surface area, which is a source of heat loss by radiation. Additional energy is required to maintain the metal temperature.
For alloys which are mildly reactive with crucibles, such as the nickel-base superalloys referred to above, a process called the "Birlec" process has been used. This process was developed by the Birmingham Electric Company in Great Britain. In the Birlec process, induction is used to melt just enough metal to pour one casting. Instead of pouring metal from the crucible conventionally, however, by tilting it and allowing the melt to flow over its lip, the crucible has an opening in its bottom covered with a "penny" or "button" of charge metal. After the charge is melted, heat transfer from the molten charge to the penny melts the penny, allowing the molten metal to fall through the opening into a waiting casting mold below.
By using a small quantity of metal with the proper induction melting frequency and power in the Birlec process, the metal can be "haystacked," or partially levitated, and held away from the crucible sides for much of the melting process, thus minimizing, although not eliminating, contact with the crucible sidewall. Such a process is in use today for the production of single crystal investment castings for the gas turbine industry. See, "From Research To Cost-Effective Directional Solidification And Single-Crystal Production.An Integrated Approach," by G. J. S. Higginbotham, Materials Science and Technology, Vol. 2, May, 1986, pp. 442-460.
The use of "haystacking" to melt refractory and titanium alloys was tried by the U.S. Army at Watertown Arsenal in the 1950s, using carbon crucibles. See, J. Zotos, P.J. Ahearn and H. M. Green, "Ductile High Strength Titanium Castings By Induction Melting", American Foundrymen's Society Transactions, Vol. 66, 1958, pp. 225-230. An attempt was made to improve on their results in the 1970s by combining the haystacking process with the Birlec process. See, T.S. Piwonka and C.R. Cook, "Induction Melting and Casting of Titanium Alloy Aircraft Components," Report AFFL-TR-72-168, 1972, Air Force Systems Command, Wright-Patterson AFB, Ohio. Neither of these attempts was successful in eliminating carbon contamination from the crucible, and there was no satisfactory method of controlling the pouring temperature of the metal to the accuracy desired for aerospace work.
In short, there has heretofore been no efficient way to melt and control pouring temperature which avoids crucible contamination. A need exists for such a way, particularly for highly reactive metals such as refractory metals and their alloys and titanium and its alloys, and for moderately reactive alloys such as nickel-based superalloys and stainless steels.